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bundled or layered top bars at support in beam

bundled or layered top bars at support in beam

bundled or layered top bars at support in beam

(OP)

The design of top bars at support of a 300mm width by 500mm depth rc beam has 7 bars. Is it recommended to put all 7 bars in one layer (top most) and bundled together in two's or distributed in two layers? What do you usually do in your practice? I think distributing it in two layers can diminish the moment capacity but with the advantage of less congestion.

RE: bundled or layered top bars at support in beam

If the bars are no greater than 1/2" in diameter and this is a concrete beam with stirrups, 7 on the bottom will work, but I would consider using pea gravel concrete.

Any larger bars, stack 'em and compute the loss. Probably one extra bar will suffice making it two layers of four stacked.

Mike McCann
MMC Engineering
http://mmcengineering.tripod.com

RE: bundled or layered top bars at support in beam

You cannot fit 7 bars in a 300mm wide beam in one layer and still provide adequate clearance for placing concrete unless it is a tee beam in which case the bars may be spread out beyond the 300mm width. Two layers would be better.

BA

RE: bundled or layered top bars at support in beam

(OP)


The bars are 20mm 7 pcs in 300mm width beam (not tee beam). If two layers are used, what is usually the separation between the two laters? I usually handle 2 to 3 bars only. Is the moment capacity percentage decrease of two layers significantly great in your experience?

RE: bundled or layered top bars at support in beam

The moment capacity is proportional to the effective depth, d. When two layers are used instead of one, the decrease in effective depth is easily calculated assuming a clearance of 25mm between bars. You should also have a clearance of 25mm horizontally between bars.

Perhaps you should consider using a larger diameter bar. Four 25mm bars will fit nicely in a 300mm wide beam.

BA

RE: bundled or layered top bars at support in beam

(OP)

Maybe the 20mm can just be bundled two together up-down or left right and put in top most position and this is better than having 25mm separation between the top and next lower layer. Have you done this bundling in practice and why not if you do not?

RE: bundled or layered top bars at support in beam

I have never used bundled bars but it is permitted by code so it could be done.

BA

RE: bundled or layered top bars at support in beam

(OP)
Why have you not used bundled bars and preferred putting them in layers? What do you think is the disadvantage of bundled bars?

RE: bundled or layered top bars at support in beam

300 mm = 11.81" width available...

2 X 1.5" clearance = 3"

2 X .375" (#3) stirrups = .75"

7 X .5" (#4) diameter bars = 3.5"

Sum = 7.25"

Net available width = 4.56"

Divided among 6 spaces = .76" between each bar (1.5X the bar diameter but 1" minimum spacing is required between the bars - I forgot that...)

BA is correct here. Stack the bars...

Mike McCann
MMC Engineering
http://mmcengineering.tripod.com

RE: bundled or layered top bars at support in beam

I have also never used bundled bars in a beam. Either use a wider beam, or bigger bars.

Mike, why convert? Sticking with mm is so much easier.

RE: bundled or layered top bars at support in beam

Quote (ticas)

Why have you not used bundled bars and preferred putting them in layers? What do you think is the disadvantage of bundled bars?

Bundled bars were not permitted when I started practicing engineering. Can't remember when bundling was introduced, somewhere around 1980 I think. Never did agree with the concept. Bond between steel and concrete, I believe, is seriously compromised by bundling. In any event, there is no need to do it so why bother?

I have always found when on site, that the concrete placers were not too happy even when I used the spacing of bars permitted by code. There is no point fighting them. Give them plenty of room to pour and vibrate the concrete. There seems to be no problem with one bar for every 4" (100mm) of beam width, so that is what I try to stick to. But in your case, I would go to 4-25M bars.

BA

RE: bundled or layered top bars at support in beam

(OP)

Are you saying even in columns, bundled bars seriously compromised bonding between bars and concrete? or is it only in beams? I commonly bundled the corners of a column

RE: bundled or layered top bars at support in beam

I seem to remember bundling bars in columns...once. But if a column needs that much reinforcement, I generally use higher strength concrete, or a larger column.

RE: bundled or layered top bars at support in beam

(OP)

Why is bundled bars in columns not a problem of bonding between bars and concrete while in beams it is a problem?

RE: bundled or layered top bars at support in beam

I didn't say that. But bars in columns are in compression, while the bars in the beams you mentioned are in tension. Bundled bars in columns are not spliced by lapping, but rather by end bearing or couplers.

RE: bundled or layered top bars at support in beam

(OP)

So the problem only occurs in the spliced sections of bundled bars? 5 of the 7 bars are just extra top bars at support. It doesn't need to be spliced to anything. There is no splice in the beams because 10 meter bars are used. So here bundled bars in beams are not problem as far as connections between bars and concrete are concerned? Or do you still think bundled bars without splice is still not good, and why is that?

RE: bundled or layered top bars at support in beam

In order to be effective, a bar must develop bond with the surrounding concrete. The five bars you mentioned may not be spliced to anything, but they must develop their tensile capacity in bond with concrete, otherwise they don't do much good. Our code in Canada, A23.3 permits bundled bars but there are special requirements for developing the bar strength in a bundle (which I am not going to outline here).

BA

RE: bundled or layered top bars at support in beam

(OP)

of course a bar normally develope bond thru the rigged surfaces of the rebars and the concrete. We used 4000 psi concrete, a single bar with rigged surfaces can bond with it, as is normally and usually the case in general construction. What I can't understand is if the bars are bundled two together, let us say two 20mm. Why is the bond between it and concrete poorer? It is the same 4000 psi concrete and vibrated as concrete is poured to the form works.

RE: bundled or layered top bars at support in beam

Just a question...since we are talking about bundling bars in a 300 wide beam. What is this beam supporting? A section detail of the beam would help, in ways you might not realize.

RE: bundled or layered top bars at support in beam

(OP)


Imagine a column and continuous beam horizontal on left and right side of the column. The middle of the left beam (and right beam) has secondary beam perpendicular to it (so the floor slabs would be only 4" in thickness). The top 7 bars at support at column came out after calculations of the moment requirements at the support. Would bundling the 7 bars in two would be good idea or can I just add another bar to make it 8 and stack it up and down with vertical distance of 25mm. But before I do this. I want to know how bundled bars become poorer in the concrete connection to the steel.

RE: bundled or layered top bars at support in beam

Spread some of the bars into the slab...but a 4" slab is tight. Why are your concrete dimensions so skinny? As to your query about bond of steel to concrete, I thought it would have been obvious that with bundled bars, there is not as much contact area available, as the concrete can't get into the area between the bars.

RE: bundled or layered top bars at support in beam

(OP)
100mm or 4" thick slab is thick already. The span is only short that is why it is not thick (because of the secondary beam). In the columns I designed. I used 3 bundles (20mm) at corners. How bad is it? Concrete is already poured into it,

RE: bundled or layered top bars at support in beam

You should still be able to spread some bars into the slab. How large is the column? When you talk about bundling bars in both columns and beams, I hope they don't clash.

RE: bundled or layered top bars at support in beam

You told us earlier it is not a tee beam. Now you are saying it is.

To keep it symmetrical, you could place 5-20M inside the beam and 1-20M in the slab on each side. Or you could place 3-20M inside the beam and 2-20M in the slab on each side. I prefer the second option because it provides better opportunity for placing and vibrating concrete around the bars.

BA

RE: bundled or layered top bars at support in beam

(OP)
not it's not a tee beam, just normal beam with floor slab connected to it. The column is 0.5x0.5. No problem with the calculations. my main concern now is I use 3 bundled bars (of three 20mm) at each corners. This is not good, how bad?

RE: bundled or layered top bars at support in beam

Your beam IS a tee beam. That is how you define a tee beam. It is a beam with a monolithically placed slab on each side. You can distribute some of the reinforcement into the slab, as BA has outlined. I am not going to say how bad bundling the beam bars would be, as I just wouldn't do it. Glad to see that your column is bigger than the beam, which allows for no clashing at the joint.

RE: bundled or layered top bars at support in beam

(OP)

My beam is a t-beam since slabs are monolithically connected to it. But is it common to put the beam reinforcement outside in the sides (of the t-beam)? I never use this. There may be some performance effect or calculation inaccuracy using this method. Also remember the 7 bars have 2 as continuous 10 meter bars (left and right beams of column) and 5 pcs as extra continuous L/3 bars at supports (top bars). So you are saying the 5 pcs can be only 3 at the beam and 2 distributed in the sides? How does this affect the performance. How common do people do this?

RE: bundled or layered top bars at support in beam

But is it common to put the beam reinforcement outside in the sides (of the t-beam)? Yes. You can provide reinforcement within the flange width of the T-Beam which is 12 times slab thickness (i.e. within a 48" strip).
I never use this. There may be some performance effect or calculation inaccuracy using this method. Not true. It is good practice. Some engineers include the temperature reinforcement in the slab for the width of the T-beam flange as part of the top reinforcement for the beam.
Also remember the 7 bars have 2 as continuous 10 meter bars (left and right beams of column) and 5 pcs as extra continuous L/3 bars at supports (top bars). Ten meters is a long bar to handle. You could use bars from each beam and lap them at the column. That gives you four top bars. You need three more, say one at center and one in the slab on each side.
So you are saying the 5 pcs can be only 3 at the beam and 2 distributed in the sides? How does this affect the performance. How common do people do this? Not very common. It is more common to lap the bars as described above.

BA

RE: bundled or layered top bars at support in beam

(OP)

BARetired, you said earlier "Our code in Canada, A23.3 permits bundled bars but there are special requirements for developing the bar strength in a bundle". Can you just give a clue what's the special requirements?

By the way, when you lap bars, it has same appearance as bundled bars! So you don't bundle bars yet you lap bars and they look the same.

RE: bundled or layered top bars at support in beam

If you are using the ACI Standard, I would think that the requirements for bundled reinforcement are spelled out in detail. I suggest you read them.

In CSA A23.3, tension lap splices require 30% more development length than that required to develop either bar. So if you know your theoretical cutoff point, you must add 1.3 times the necessary development length to each of the lapped bars.

BA

RE: bundled or layered top bars at support in beam

(OP)
BARetired wrote: "The moment capacity is proportional to the effective depth, d. When two layers are used instead of one, the decrease in effective depth is easily calculated assuming a clearance of 25mm between bars. You should also have a clearance of 25mm horizontally between bars.".

I'll choose this option of two layers. Only 4 bars can fit at the top of the 0.3m width due to the stirrups 10mm diameter and Four 20mm. Assume the depth of the beam is 0.5m. Putting the second layer 25mm below the first one is like having a beam depth combination (and moment capacity) of 0.5m and 0.475m, isn't it. Know any rule of thumb of knowing whether to adding one 20mm bar to the second layer is enough to compensate?

RE: bundled or layered top bars at support in beam

The longitudinal reinforcement will not be all the way into the corner of stirrups. They will ride the radius, moving them in or down/up, changing the available depth or width, or both.

I seldom see bundled bars. The main focus for development is to be sure of proper consolidation and good concrete mix design. Using a polycarboxylate to help the paste flow and keep aggregates in suspension with more fluid concrete. The interlock of deformations on one bar to those on the adjacent bar work with the paste and with the surrounding concrete to develop the reinforcement.

With any very large bar or bundle (i.e., large area of steel) the surrounding concrete is asked to do quite a lot. The local compressive and tensile forces in the concrete can be quite high, including splitting forces.

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